1. Field of the Invention
The present invention relates to the fabrication of integrated circuits and the deposition of film layers over a semiconductor substrate. More specifically, the present invention relates to an improved chemical vapor deposition method and apparatus for depositing and treating a titanium layer having improved sheet resistance uniformity and excellent bottom coverage at contacts.
2. Description of the Background Art
One of the primary steps in fabricating modern semiconductor devices is forming various layers, including dielectric layers and metal layers, on a semiconductor substrate. As is well known, these layers can be deposited by chemical vapor deposition (CVD) or physical vapor deposition (PVD) among other methods. In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions take place to produce a desired film. In a conventional plasma CVD process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film. In general, reaction rates in thermal and plasma processes may be controlled by controlling one or more of the following: temperature, pressure, plasma density, reactant gas flow rate, power frequency, power levels, chamber physical geometry, and others.
Increasingly stringent requirements for fabricating these high integration devices are needed and conventional substrate processing systems are becoming inadequate to meet these requirements. Additionally, as device designs evolve, more advanced capabilities are required in substrate processing systems used to deposit films having the properties that are required to implement these devices. For example, the use of titanium is increasingly being incorporated into integrated circuit fabrication processes. Titanium has many desirable characteristics for use in a semiconductor device. Titanium can act as a diffusion barrier between, for example, a gold bonding pad and a semiconductor, to prevent migration of one atomic species into the next. Also, titanium can be used to improve the adhesion between two layers, such as between silicon and aluminum. Further, use of titanium, which forms titanium silicide (TiSix) when alloyed with silicon, can enable, for example, formation of ohmic contacts. One common type of deposition system used for depositing such a titanium film is a titanium sputtering or physical vapor deposition (PVD) system. Such sputtering systems are often inadequate, however, for forming devices with higher processing and manufacturing requirements. Specifically, sputtering may damage previously deposited layers and structures in such devices creating performance and/or yield problems. Also, titanium sputtering systems may be unable to deposit uniform conformal layers in high aspect ratio gaps because of shadowing effects that occur with sputtering.
In contrast to sputtering systems, a plasma-enhanced chemical vapor deposition (PECVD) system may be more suitable for forming a titanium film on substrates with high aspect ratio gaps. As is well known, a plasma, which is a mixture of ions and gas molecules, may be formed by applying energy, such as radio frequency (RF) energy, to a process gas in the deposition chamber under the appropriate conditions, for example, chamber pressure, temperature, RF power, and others. The plasma reaches a threshold density to form a self-sustaining condition, known as forming a glow discharge (often referred to as xe2x80x9cstrikingxe2x80x9d or xe2x80x9cignitingxe2x80x9d the plasma). This RF energy raises the energy state of molecules in the process gas and forms ionic species from the molecules. Both the energized molecules and ionic species are typically more reactive than the process gas, and hence more likely to form the desired film. Advantageously, the plasma also enhances the mobility of reactive species across the surface of the substrate as the titanium film forms, and results in films exhibiting good gap filling capability.
One known CVD method of depositing titanium films includes forming a plasma from a process gas that includes a TiCl4 source gas and a hydrogen (H2) reactant gas in a standard PECVD process. Such TiCl4/H2 PECVD processes result in the deposition of a titanium film that has good via-fill, uniformity and contact resistance properties making the film appropriate for use in the fabrication of many different commercially available integrated circuits. However, the high energy and temperatures associated with PECVD also increases the reaction rate of contaminants such as carbon and oxygen at the wafer surface during the deposition process. Additionally, surface titanium may react (oxidize) with ambient oxygen as a wafer is transferred between process chambers. As such, the deposited titanium may contain impurities which alter (increase) the resistance of the deposited layer and render devices constructed therefrom defective or inoperable.
Therefore, there is a need in the art for a suitable method of treating deposited layers such as titanium so as to protect them from contaminants during the fabrication process.
The present invention provides an improved CVD deposition and treatment process for titanium films. According to the method of the present invention, a passivating layer for a titanium layer that has been deposited on a substrate in a reaction chamber is formed by adding a flow of hydrogen and a flow of nitrogen to the chamber. The flows of hydrogen and nitrogen are approximately 800 sccm and continue for approximately 10-30 seconds respectively. The method may further comprise the step of forming a nitrogen plasma in the chamber for approximately 10 seconds wherein such case the flows of hydrogen and nitrogen continue for approximately 8 seconds respectively. The plasma is formed by applying RF power to an electrode located within said chamber or by a remote plasma source and channeled to said reactor chamber. Alternately, the passivation layer may be formed just by using a nitrogen plama alone for approximately 10-30 seconds at the same RF power level. The plasma in either case may further comprise hydrogen and argon and the layer of titanium has been deposited by CVD.
Additionally, in a semiconductor wafer processing system comprising a reactor chamber for processing a semiconductor wafer onto which a layer of titanium has been deposited and a processor for controlling the operation of said reactor chamber, a processor readable medium containing a program that, when executed by said processor, causes said reactor chamber to passivate said layer of titanium by adding a flow of nitrogen and a flow of hydrogen to said reactor chamber in the presence of said semiconductor wafer. The processor readable medium further contains a program that, when executed by said processor, causes the reactor chamber to passivate said layer of titanium by forming a nitrogen plasma in said reactor chamber in the presence of said semiconductor wafer wherein said flow of nitrogen and hydrogen continues for approximately 10-30 seconds.
In accordance with the method of the present invention, a passivation layer is formed over the titanium layer, for example a titanium nitride layer. The passivation layer coats the titanium thereby reducing the likelihood of contamination by byproducts of the deposition process or ambient oxygen or similar reactants that may otherwise attack and alter the stability of the resultant deposited film.